1. Field of the Invention
The present invention relates to a fluorine gas generator, in particular a fluorine gas generator capable of generating high-purity fluorine gas having very low impurity content and suited for use in the process of manufacturing semiconductors, among others.
2. Description of the Relating Art
Conventionally, fluorine gas is one of the key gases essential in the field of semiconductor production, for instance. While it is used as such in certain instances, the demand for nitrogen trifluoride gas (hereinafter referred to as “NF3 gas”) and like gases synthesized based on fluorine gas and intended for use as cleaning gases or dry etching gases in semiconductor manufacturing apparatus has been rapidly increasing. Further, neon fluoride gas (hereinafter referred to as “NeF gas”), argon fluoride gas (hereinafter referred to as “ArF gas”), krypton fluoride gas (hereinafter referred to as “KrF gas”) and the like are excimer laser oscillation gases used in patterning of integrated semiconductor circuits, and the raw materials thereof used in many cases are mixed gases composed of a rare gas and gaseous fluorine.
The fluorine gas or NF3 gas for use in the manufacture of semiconductors and the like is required to be highly pure with the impurity content as low as possible. On the sites of semiconductor manufacture, for instance, necessary amounts of fluorine gas are taken out of gas cylinders filled with fluorine gas. It thus becomes very important to secure sites for storing such cylinders, store the gas safely, maintain the purity of the gas, and manage for such purposes. As for NF3 gas, for which the demand has been rapidly increasing lately, the demand tends to exceed the supply, hence there arises a problem that certain amounts of the gas should be in stock. Further, NF3 gas has been changed into fluorine gas for the global warming. In view of these, to have a fluorine gas generator or producer of the on-demand and on-site type at the site of use thereof is preferred to handling high-pressure fluorine gas storing such cylinders.
Conventionally, fluorine gas is produced in an electrolytic cell such as shown in FIG. 3. The electrolytic cell body 201 is generally made of Ni, Monel, carbon steel or the like. Further, in case that the bottom of the electrolytic cell body serves as cathode 201, a bottom plate 212 made of polytetrafluoroethylene or the like having electric insulating and corrosion resistant property is disposed for preventing the hydrogen gas and fluorine gas generated from being mixed with each other. The electrolytic cell body 201 is filled with an electrolytic bath 202, namely a potassium fluoride-hydrogen fluoride system (hereinafter referred to as “KF-HF system”) in the form of a mixed molten salt. The cell or bath is divided into an anode chamber or compartment 210 and a cathode chamber or compartment 211 by means of a skirt 209 made of Monel or the like. Upon applying a voltage between a carbon or nickel (hereinafter referred to as “Ni”) anode 203 contained in the anode chamber 210 and a Ni or iron cathode 204 contained in the cathode chamber 211, electrolysis occurs and fluorine gas is generated. The fluorine gas generated in the anode chamber 210 is discharged through a product line 208, while the hydrogen gas generated in the cathode chamber 211 is discharged through a hydrogen gas discharge line 207 (cf. e.g. Patent Document 1: Laid-open Japanese Patent Application (J P Kohyo) H09-505853).
However, when electrolysis is interrupted in the prior art fluorine gas generators, current supply between anode 203 and cathode 204 is stopped, and the fluorine gas remaining in the anode chamber 210 is then adsorbed into the anode 203, with the result that the pressure in the anode chamber 210 drops. This phenomenon occurs more remarkably in cases where the anode 203 is made of carbon. The problem is that when the pressure within the anode chamber 210 drops, not only the electrolytic bath liquid level in the anode chamber 210 rises, the liquid level in the cathode chamber drops, but the condition of liquid level in anode chamber 210 and cathode chamber 211 is uneven and the electrolysis restarting condition is unstable. In the worst case, hydrogen gas generated goes below the partition wall 209 and mixed with fluorine gas and explodes.
Accordingly, it is an object of the present invention, which has been made in view of the above problems, to provide a fluorine gas generator in which the fluorine gas discharge port disposed in the anode chamber of the fluorine gas generator can be closed and the electrolytic bath liquid level in the elocrtolyzer can be controlled even during the period of suspension of fluorine gas generation.